In inkjet printing, oxygen inhibition occurs during the curing step; if there is oxygen present at the surface, the oxygen can penetrate into the surface and interfere with the radical polymerization, leaving unreacted monomers and oligomers at the surface. This is the tack that some people feel as they rub a finger across the surface, and may get traces of wet residue on their gloved hand, which is undesirable.
One practice for formulating UV curable inks for inkjet printers is to use photoinitiators with a free radical initiation wavelength range of 250-350 nm in order to overcome oxygen inhibition on the surface of ink films and to achieve good surface curing.
Ultra-violet light-emitting-diode (UV LED) technology offers a number of advantages compared with traditional mercury arc lamps in the curable ink printing environment. However, most cost effective UV LED lamps with adequate power for UV inkjet printing systems are in the range of 375-405 nm in wavelength. Currently, UV LED curing lamp systems lack wavelengths in the range of 250-350 nm and thus do not have the ability to initiate reaction by photoinitiators that require 250-350 nm for photoinitiation. Therefore, UV curable inkjet inks cured with UV LED lamps have poor surface curing due to oxygen inhibition. A current UV LED curable inkjet ink system suffers poor surface cure unless inert gas is used to displace oxygen prior to curing.
Several methods are used to obtain good surface cure for a UV LED lamp with wavelengths of 375-405 nm. Nitrogen inerting or blanketing on the printing areas is an effective way to achieve good surface curing by mitigating oxygen inhibition. However, such approach requires costly and bulky nitrogen generators as well as compressed air, which increases printer cost and adds cost to customers in terms of compressor capacity.
Inkjet inks have very low viscosity, which makes them susceptible to oxygen inhibition due to oxygen diffusion. Thus, one common practice in UV LED ink formulations is to use the combination of multifunctional monomers and additives, such as amine, thiol, ether, etc., to improve the surface curing. Increasing the amount of multi-functional monomers or oligomers can promote crosslinking to reduce the surface tack. However, this approach results in a hard and brittle ink film, which is not suitable for many applications that require flexible ink film. Increasing photoinitiator concentration or LED lamp power have also been used to reduce the surface tack.
The current available technology cannot completely overcome oxygen inhibition and cannot achieve tack-free surface curing, without nitrogen blanketing.
There is a need for a radiation curable ink composition that provides flexible ink film and achieves tack-free surface cure without a need of nitrogen blanketing or expensive LED lamps.